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Model Wind Tunnel 



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Model Wind Tunnel 



Written By: Doug Desrochers 



f TOOLS: 

Dremel rotary tool (1) 
Drill (1) 

Hot glue gun (1) 
Jigsaw (1) 
Screwdriver (1) 
Soldering materials (1) 
Wire cutters (1) 



© PARTS: 



Household fan (1) 

preferably strong and mult is peed 

Lexan panel (1) 

$34 at Home Depot stores. Similar 
polycarbonate plastics can be found at 
TAP Plastics, tapplastics.com. Acrylic is 
cheaper, but not as easy to cut. 

Digital pocket scale (1) 
$15 

Cabinet handle (1) 

Hinges (2) 

Utility hinges (4) 
with screws 

Steel wire (1) 

such as fiberglass insulation support 

wire 

Hook-up wire (1) 

Plywood (4" pieces) 

or equivalent in 1x8. 1x12. and/or 1x4 

boards 



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Model Wind Tunnel 



Plywood (scrap) 

Wood screws (30) 

Wood glue (1) 
and plastic epoxy 

Taped) 

Cardboard box (scrap) 
or thin plastic 

LED (10 strand) 
(optional) 

Drinking straws (box) 
(optional) for flow straightener 



SUMMARY 

Last year, at the annual Pinewood Derby race for our local Cub Scout pack, we used a 
Matchbox radar gun (see MAKE, Volume 10, page 148) to measure the top speeds of 
several cars. We learned that they reached 10mph-1 1mph at the bottom of the slope. Armed 
with this knowledge, along with some Lexan plastic, a fan, and a precise digital scale, it was 
time to build a wind tunnel for this year's event. 

I wanted to keep all the elements of the tunnel visible, so the Scouts could have fun seeing 
and learning how it worked. It's a simple, open-loop type of tunnel powered by a household 
fan at the exhaust end (having the fan blow air into the tunnel would generate too much 
turbulence). 

The highlight of this project is the test stand and its very accurate drag gauge. The stand 
holds test objects in the middle of the tunnel, and is supported by 2 hinged struts that pivot 
backward. When wind pushes against the object, the struts move a back plate that pushes 
into a force beam, which measures the force. The force beam is hacked out of an 
inexpensive digital scale that measures down to 0.1 gram. 



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Model Wind Tunnel 




I used 7/8" plywood, 8"x40", for the tunnel's floorboard. Paint it to make it smooth. Cut a 
3"x7" rectangle out of the center for the test section: first drill pilot holes, then cut it out 
with a jigsaw. 

Make a base the same length as the floorboard but several inches wider, to give extra 
room for the control panel. I used a 1" board for the base. Line up the boards and trace the 
3"x7" hole onto the base — this will help you align the test section later. 



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Model Wind Tunnel 




• The upper surface of the test stand must be flush with the floor of the tunnel, and the 
hinged struts should pivot easily and balance just aft of vertical. All other connections 
must be rigid; minute movements of the stand must be applied to the force beam rather 
than absorbed in any joints. 

• Using scrap W plywood, cut two 2"-square vertical struts and a 2 3 /4"x6 1 /2" test section 
floor. Cut the back plate 2"x2 1 /2" tall, and lighten it with a large cutout, because weight not 
centered in the pivoting mechanism will tend to fall backward and inflate very small 
measurements. Glue a V2"x 3 A" block to the lower-center-back of the plate; this spacer is 
what presses against the force beam. 

• Mount the underside of the test section floor to the struts, using two IV2" hinges. Glue the 
back plate to the aft end, and brace the connection diagonally with stiff steel wire on either 
side. I looped one end of each wire, screwed it next to the front strut, and bent the other 
end up to fit into a hole drilled in the bottom end of the back plate. 

• Use 2 more hinges to mount the struts to the base board, making sure the stand pivots 
smoothly. A reliable mechanism requires tight tolerances! I needed a snub Phillips 
screwdriver to handle these lower hinges. 



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Model Wind Tunnel 







Unscrew the back of the Cen-Tech scale, then remove and save the screws holding the 
force beam. Carefully cut away the hot glue over the battery and force beam wires, and 
use wire cutters to cut an opening through the side of the case big enough to pull the beam 
through, keeping its wires intact. Use a Dremel to cut the frame above the display, not too 
deep, and toss the excess. Also cut off the battery compartment; I lengthened the power 
wires for more mounting options and duct-taped them down later for strain relief. 

Cut a wood block about 1"x2"x 1 /2" and mount the force beam to this block, using the saved 
screws. Now, the critical part: lean the stand back slightly and position the force beam 
block on the base such that the spacer just touches the force beam near the sticker that 
says "500g." Hold the block in place, power up the scale, and confirm that it responds 
correctly before screwing the block to the base board. If the scale reads negative values, 
flip the beam around. 




The rest is easy. Cut 4 support blocks sized to lift the floorboard so that its top sits flush 
with the top of the test stand. I used 2x4 scraps, shaped the middle blocks with a ledge for 
mounting the control panel, and attached them with wood screws. 



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Model Wind Tunnel 




Cut 2 pieces of 0.220" Lexan 7"x30" for the tunnel sides, and 1 piece 74"x30" for the top. 
Cut a test section access door in 1 side. If you're adding LED lighting, the door should 
clear the floor by about 1". 

Mount the door with the brass hinges along the top, and attach the handle at the bottom. 
Air should not leak in around the door during use, so you need to seal it; clear tape works 
for low velocities, but Lexan is sturdier and better looking. One lesson learned: glue a 
small block of Lexan inside the door opening to prevent the door from swinging inward. 

Finally, assemble the tunnel by screwing the Lexan sides to the floorboard and using 
plastic epoxy to secure the top piece. 

Below the access door, create a control panel to house the scale display, fan controls, and 
LED switch. I used Lexan for better visibility. For the LEDs, I hot-glued 7 of the 10 along 
the floor near the test section and taped the remaining 3 under the test section, to 
illuminate the tunnel's workings. 



Step 6 — Mount the fan. 

• Crack open the fan, extend all its control wires about 2', and remove any unneeded 
supports. Mount the fan as near as possible to the center of the tunnel, blowing out the 
back. Seal around it to close any holes between it and the tunnel. I used thin scrap plastic 
from toy packaging, to preserve the view, but cardboard and duct tape would work fine. 
Regardless, use tape and hot glue to make the seal complete. 



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Model Wind Tunnel 




Floorboard Exhaust fan 




Make a cardboard sleeve with the same cross- section as the tunnel, and pack it with 2" 
pieces of drinking straws — this part goes faster with a small- fingered helper. The straws 
reduce turbulence, but they also cut the wind speed by several mph, and the tunnel will 
work without them. To improve performance, add a cardboard airscoop in front of the 
straightener. That's it! 



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Model Wind Tunnel 





• Make sure the tunnel is level and 
stable, for repeatable drag 
numbers. Tape or block your car 
wheels on the test section floor. 

• Now fire up the fan to test your car 
— the force in the display is very 
close to the actual drag caused by 
the car (set it to "grams" for higher 
resolution). Modify your shape to 
minimize that drag. Now set track 
records at the Pinewood Derby! 

• For airflow visibility, run a 
Halloween fog generator at the 
intake and take flash photos to 
capture the stream lines. 

• You can measure the wind speed 
with a mini- anemometer — eBay 
is a good source. 



This project first appeared in MAKE Volume 15 , page 143, 



This document was last generated on 201 2-1 1 -02 09:24:23 AM. 



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